Jack's a villain (5)
I believe the answer is:
knave
I believe this clue is a double definition.
'jack's' is the first definition.
('knave' can be a synonym of 'jack')
'a villain' is the second definition.
(knave is a kind of villain)
(Other definitions for knave that I've seen before include "Scoundrel - card" , "Jack - the cad!" , "A swindler" , "Varlet; Jack" , "Rat" .)